The present invention is generally directed to a metal to silicon contact system for use in silicon integrated circuits for VLSI (Very Large Scale Integration). More particularly, the present invention relates to the use of double layer refractory metals to simultaneously achieve good ohmic contact and conductivity both of which are desirable for high production yields in modern VLSI production technology. However, the present invention is also applicable to semiconductor devices generally and particularly in power semiconductor devices under conditions of high temperature and high current.
VLSI circuits typically require contact to a shallow depth junction (often shallower than 0.3.mu.). A VLSI CMOS (complementary metal-oxide-semiconductor) contact system must make good ohmic contact with p.sup.+ silicon, with n.sup.+ silicon and with polysilicon gates. Additional requirements for a contact metallization system include: (1) high thermal stability; (2) good adhesion to Si and to SiO.sub.2 ; (3) high reliability (no junction spiking, for example); (4) high interconnection conductivity; (5) low electromigration; (6) good step coverage; and (7) smooth surface integrity for multilevel processing.
Aluminum is used extensively as a general-purpose metallization material since it makes good ohmic contact and has high conductivity. The inherent problems of aluminum, such as spiking and electromigration, however, emphasize a need for alternate metallization systems. Alusil (e.g., Al with 1% Si by weight) reduces spiking failures and further additions of Cu or Ti reduce electromigration. These mixed alloys do not, however, completely eliminate all of the aforementioned problems; and aluminum is generally suitable only for devices having large junction depths, that is, depths greater than or equal to 0.5 microns. In order to eliminate spiking, platinum silicide or Ti/W alloy barriers are used in combination with aluminum. In these cases, alloy or platinum silicide layers make good ohmic contact and the aluminum provides high conductivity interconnections. The penetration of aluminum into the silicon junction (through the silicide which is not a good aluminum barrier layer) is prevented by the insertion of an alloy barrier layer (for example, titanium/tungsten) between aluminum and platinum silicide layers. These process complications and the difficulties associated with removal of the unreacted platinum after siliciding, however, reduce the desirability of this system. Other metals and their combinations with silicide are also employed. However, none of them satisfies all of the above-mentioned features for a desirable VLSI contact system.
Titanium is used as a contact material in conjunction with aluminum. However, thermal instability is a major problem in this system. Aluminum reacts with Ti at a temperature of about 400.degree. C. to form Al.sub.3 Ti. Titanium reacts with silicon at a temperature of about 500.degree. C. to form TiSi and at 600.degree. C. to form TiSi.sub.2. Since extensive siliciding degrades a shallow VLSI junction, titanium silicide thickness must be limited. Silicon also tends to diffuse through the thin TiSi.sub.2 to the Al layer, thus causing degradation as a result of Al electromigration properties. Al also diffuses through thin TiSi.sub.2 and thus penetrates into silicon to degrade the junction characteristics.
The system proposed herein does not produce such thermal instability problems, is simpler and is not afflicted with limitations posed by the systems illustrated above.